1. Field of the Invention
The present invention relates to cold pilgering of thin-walled metallic tubing and, more particularly, is concerned with high strength cemented carbide tooling, i.e. dies and mandrels, for use in a pilgering machine.
2. Description of the Prior Art
Cold-pilgering is a conventional process by which a tube is advanced over a stationary mandrel and simultaneously compressed using two opposing roller dies resulting in the reduction of the cross-sectional area and in elongation of the tube. Representative of the prior art pilgering machines are the ones disclosed and illustrated in U.S. Pat. Nos. to Arrington (3,416,346), Edstrom et al (3,487,675 and 3,690,850), Naylor et al (4,090,386) and Matinlassi (4,233,834).
Typically, the input tube is reduced and elongated to the final tube by passing through a succession of stations of the cold-pilgering machine with each station being composed of a stationary mandrel/roller die set. Reduction is effected in both the diameter and wall thickness of the tube by means of the tapered shape of the mandrel and the circumferential tapered shape of grooves in the dies which embrace the tube from above and below the mandrel and roll in a constant cycle back and forth along the tube. Between each cycle of die movement, the tube is advanced and rotated incrementally along the mandrel. The mandrel prevents the tube from collapsing under the force of the roller dies while at the same time dictates the inner diameter of the tube.
Although the mandrels and roller dies are fabricated from high strength steel, a limiting factor in the cold-pilgering process is the need for frequent replacement of mandrels and roller dies. Mandrel replacement is required when the steel mandrels become overstressed and break from the severe operating conditions and the occasional bending moments imposed thereon by tube eccentricity or slight misalignment. Also, the mandrels must be remachined occasionally to remove metal buildup caused by general use. Roller die replacement is frequently required due to occurrence of surface cracks, fretting and spalling in the die grooves of the steel dies as a result of the severe operating conditions of the pilgering machine. The roller dies also must be remachined occasionally to remove metal buildup caused through general use. Typically, a high strength steel roller die will only produce approximately 20,000 feet of tube before the grooves must be remachined.
Consequently, a need exists to increase the longevity of the pilger tooling, i.e., mandrels and roller dies, so as to improve the productivity and efficiency of the pilgering machine.